The _d_switch_error function has been removed from druntime.

The `__ArrayCast` lowering isn't (always?) done when casting a static
array to a slice with different element size.
So exploit constant source lengths and compute the target length at
compile-time.

This fixes compilable/ldc_github_421.d - a float[16] can be cast to a
float4[] with constant length 4.

Invoke _d_newThrowable for exception allocation & initialization, and
increase the reference count in LDC-specific _d_throw_exception for MSVC
targets. Fixes runnable/test19317.d.

By converting the real_t to a `double` and repainting that value.
E.g., this fixes druntime-test-hash for MSVC targets;
core.internal.hash.hashOf!(real) reinterprets a 64-bit (target) `real`
as `ulong`, and that code path is taken at CTFE too.

expressionSemantic() for a ThisExp doesn't just set its type (and do
nothing if it's already set), but may also set the ThisExp.var
VarDeclaration. This crucial step was previously skipped after
syntaxCopying an already analyzed ThisExp, as a later
expressionSemantic() for that copy was a no-op and `var` still pointed
to the original one.

This fixes runnable/test19731.d, as dlang/dmd#9467 may lead to a
syntaxCopy of a sema3'd FuncDeclaration, and that copy replacing the
original function. A ThisExp's `var` still pointed to the variable in
the original function, so LDC assumed it was a nested variable and
failed to find a matching parent function.